The present invention concerns a forming tool for making structures in flat material, especially in plate glass, which comprises a base tool and a separate forming tool arranged on the base tool and provided with a structuring surface.
Plate glass provided with large-scale precision structures is required for precision applications, especially in the optical glass field. This type of glass is used, for example, for display panels of new generation, flat-display-screen devices (Plasma Display Panels (PDP), Plasma Addressed Liquid Crystal (PALC)). Micro-channel structures for control of individual lines or columns, which extend over the entire active display screen width or height and in which a plasma is ignited by electric discharge, are provided in this flat display screen glass. The boundary of an individual channel on both sides is provided by rectangular crosspieces whose width is as small as possible (i.e. &lt;100 .mu.m). In order to obtain a sufficient discharge volume, the height of the crosspiece is substantially larger than its width. The spacing of the crosspieces should be as small as possible. Currently typical values of between 360 .mu.m and 640 .mu..mu.m are achieved in small scale production. The height of the crosspieces amounts to from about 150 .mu.m to 250 .mu.m with a width of from 50 .mu.m to 100 .mu.m.
During structuring of this flat display screen glass, which for example is a 25"-PALC screen of a size of 360 mm.times.650 mm, the exact lateral dimensioning, relative positioning and reproducibility of the channels and thus the stability of the forming tool are crucial because of the later positioning of the electrodes. With a method based on hot shaping by means of a conventional chromium-nickel-steel tool, the thermal expansion coefficient amounts to about 12.times.10.sup.-6 /K. For example, for a tool length of about 360 mm, as required for a 25" PALC display screen, this always causes a length change of about 4 .mu.m per K temperature fluctuation. Considering that the required positioning accuracy of the electrodes in the micro-channels is in the range of .+-.10 .mu.m, a temperature fluctuation of .+-.2.5 K can cause considerable problems. The permissible temperature fluctuations are considerably reduced in the larger display screens; for example 42" display screens.
The problems are similar with other applications of plate glass with large-scale precision structures.
The structures in plate glass are formed in two standard ways with the above-described forming tool. The first way is designated as hot forming. In hot forming the heated forming tool with its structuring surface is pressed on one side of the plate glass in the heated glass material and the structures are formed in the glass surface.
In the second way cavities or recesses in the structuring surface of the forming tool are filled with a paste-like material, and the forming tool is applied to a glass surface so that this paste-like material comes into contact with the glass surface and is hardened. This customarily happens with suitable heating.
A comparatively high tool wear occurs on the structuring surface in both ways however, which requires continual replacement of the entire expensive forming tool with a considerable increase in maintenance expense in the case of small structure radii.
Forming tools are described in German Patent DE-PS 111 216 and U.S. Pat. No. 1,261,939, which avoid a continual expensive exchange of the complete forming tool on account of tool wear on its structuring surfaces.
These patent documents describe a forming tool with a structuring surface for formation of structures in plate glass, which comprises a base tool (in the case of DE-PS 111 216 rollers or rollers and/or press plate, in the case of U.S. Pat. No. 1,261,939, rollers) and a separate forming member arranged on the base tool. The separate forming member is provided with the structuring surface (in the case of DE-Patent it is a web or fabric or mesh and in the case of the US-PS, plates 196).
In the case of excessive wear only the separate forming member must be replaced with comparatively small maintenance expense, so that the tool and manufacturing costs for the structured plate glass are reduced.
In the German Patent document the rollers or press plates typically are made from metal. These rollers or press plates thus have sufficiently large thermal conductivity and a comparatively large thermal expansion coefficient, while in contrast the forming member, namely the web or mesh, has a comparatively very small thermal conductivity with comparatively small thermal expansion coefficient. This is also true for the rollers and plates of the U.S. patent, which are both typically made from metal.
In order to perform a hot forming with the known forming tool, the entire forming tool must disadvantageously be heated with an accompanying high-energy consumption and expense. Since the base tool has a comparatively high thermal conductivity and thermal expansion coefficient, the known forming tools experience a comparatively large thermal expansion effect.